JPH04309330A - Nuclear magnetic resonance image pickup device - Google Patents

Abstract

PURPOSE:To negate the influence of ununiformity of a magnetic field by a stimulated echo method, and also, to execute the transverse relaxation (T2) emphasis, and moreover, to execute the data collection in a short time, in the nuclear magnetic resonance image pickup device using a preparation pulse. CONSTITUTION:Magnetization is laid down to a plane vertical to the static magnetic field direction, rotated by l80 deg.C after the time tau1, and subsequently, after tau2=tau1, an inclined magnetic field GST is applied to the magnetization collected in one direction on the plane vertical to the static magnetic field direction, and after tau3 time, a preparation pulse for rotating the magnetization by 90 deg. by an RF pulse is given to a body to be examined, and thereafter, a read pulse is given to the body to be examined plural times, and at every read pulse thereof, nuclear magnetic resonance information radiated from the body to be examined is obtained. By lengthening tau1+tau2 so that the sufficient transverse relaxation emphasis can be obtained and making a read time tau4 equal to 3t3 profits of the stimulated echo method are obtained.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention provides a preparation pulse to a subject, then a plurality of read pulses to the subject, and for each read pulse, the nuclear magnetic resonance signal emitted from the subject is detected. The present invention relates to a nuclear magnetic resonance imaging device that collects nuclear magnetic resonance information such as image information or spectral information.

0002

2. Description of the Related Art An image forming apparatus using nuclear magnetic resonance (hereinafter referred to as NMR) is disclosed in, for example, Japanese Patent Laid-Open No. 57-6347,
This is shown in the magazine ``Image Diagnosis'', Vol. 12, No. 1, pp. 20-42, published by Shujunsha in January 1980. For example, as shown in FIG. 5, an image forming apparatus using the NMR method includes a static magnetic field generating means 11.
A static magnetic field parallel to the Z-axis is generated, and the magnetic field strength is tilted in the X-axis, Y-axis, and Z-axis directions by the gradient magnetic field generating means 12, and gradient magnetic fields GR, GS, and GP whose directions are in the Z-axis direction are generated. The subject 13 is placed in a space where these static magnetic fields and gradient magnetic fields are generated. Note that the three gradient magnetic fields only need to have the gradient directions of their magnetic field strengths intersect with each other other than on the same plane, and do not necessarily need to intersect at right angles. In this way, spatial information from the subject 13 can be discriminated.

A high-frequency pulse current is applied to the transmitting/receiving coil 14 to apply a high-frequency magnetic field to the subject 13 in a direction perpendicular to the Z-axis. The NMR signal generated at that time from the subject 13 is received by the coil 14, the received output is amplified and detected by the high frequency signal transmitter/receiver 15, the detected output is sampled and converted to a digital signal by the AD converter 16, and the The signal is input to a signal processing device 17 consisting of a computer.

By the way, NMR relies on the fact that when certain atomic nuclei are placed in a magnetic field, they precess around the direction of the applied magnetic field at a frequency proportional to the strength of the magnetic field. This frequency is known as the Larmor frequency and is given by ω0 = γH0. However, γ is the gyromagnetic ratio of the atomic nucleus, and H0 is the magnetic strength. When a magnetic field whose strength varies along a certain direction, a so-called gradient magnetic field, is applied, atomic nuclei (hereinafter referred to as nuclear spins) at different positions in that direction precess at different frequencies.

[0005] There are roughly two methods for obtaining positional information of resonant nuclear spins using this property, that is, information indicating from which position in the subject 13 the NMR signal originates. One method is to apply a gradient magnetic field to a certain axis and keep its strength constant over time, during which time the NMR
This method collects signal data and uses frequency encoding to directly obtain information on the position of nuclear spins along this axis. The other method is to apply a gradient magnetic field to a certain axis and sequentially change the time-integrated value of its intensity, that is, after changing the initial phase of precession, collect NMR signal data. This method uses phase encoding to obtain positional information of nuclear spins along this axis. In either case, there are several possible calculation methods for obtaining position information from signal data, but the Fourier transform method is most commonly used today, so the following explanation uses this as an example. Do the following. In the case of the Fourier transform method, data is basically converted from the time axis to the frequency axis, so the data before calculation processing must be a temporally ordered signal sequence to be meaningful. For this reason, in the case of frequency encoding, the gradient magnetic field strength applied to a certain axis is held constant during signal data collection, and in the case of phase encoding, the time integral value of the gradient magnetic field strength applied to a certain axis is is being changed sequentially. That is, in both frequency encoding and phase encoding, the integral value of the strength of the gradient magnetic field up to the sampling point and the time of applying the magnetic field is made to change successively for each sampling. If such data processing is performed, by directly Fourier transforming the obtained signal data series, the position information of the nuclear spin along a certain axis can be obtained from NM
It is easy to understand that information obtained from the R signal, such as the spatial distribution of nuclear spin density, relaxation time, etc., can be obtained. The arithmetic processing unit 17a in the signal processing device 17 performs arithmetic processing to obtain such spatial distribution, that is, image information, and displays the obtained image information on the display 18 as an image.

There is a spin warp method as a method for obtaining an NMR signal from which positional information of nuclear spins can be determined (for example, see the above-mentioned magazine). For example, by the spin warp method,
To obtain a two-dimensional image in the Z plane, a gradient magnetic field GS for selecting a slice (cut plane) tilted along the Y axis is applied in period 5 (periods 1 to 4 will be described later) as shown in FIG. A high-frequency magnetic field pulse for reading is applied to a specific atomic nucleus in a cross section 13a (FIG. 7) of the object 13 taken through the object 13 at a position on the Y axis determined according to the carrier frequency of the reading pulse. selectively excite (the gradient magnetic field GS for slice selection selects the position of the cross section 13a), and following this excitation, in period 6, a magnetic field GSr for rephasing in the opposite direction to the gradient magnetic field GS for slice selection is applied, and for reading. Applying a dephasing magnetic field GRd in the opposite direction to the gradient magnetic field GR,
Reading gradient magnetic field GR tilted along the X axis during period 7
, receive the NMR signal at this time, and convert it to, for example, 2
By sampling 56 times at a constant period and frequency decoding the sampling time series, that is, 2
By performing 56-point Fourier transformation, X in the cross section 13a
Nuclear spin position information along the axis (the frequency represents the position on the X axis, and the amplitude at that position represents the vector sum of the nuclear spins in the Z direction) is obtained.

Between the selective excitation and reading, that is, during period 6, a phase gradient magnetic field GP tilted along the Z-axis is applied in a pulsed manner, and its amplitude is changed every time a sampling time series is obtained. Change each time at equal intervals (stepwise). For example, if this amplitude change is set to 256 steps,
In other words, reception by 256 samplings is repeated 256 times, and the image field of view is -a<=x<a, -a<=z
<a, then apply a magnetic field GP of such intensity that the phase of the nuclear spin at z=a advances by (n-129)π with respect to the phase of the nuclear spin at z=0 when transmitting and receiving the n-th read pulse. give. In this way, each of the 256 GP pulses maximizes the response at each position in the Z-axis direction for each of the 256 spatial frequencies (corresponding to positions on the X-axis).

[0008] The signal obtained as described above is subjected to two-dimensional Fourier transform to obtain an image of 256×256 pixels. In this way, in order to match each position and phase in the Z-axis direction, the width of the GP pulse may be changed sequentially instead of changing the amplitude of the GP pulse sequentially (stepwise). However, the product of both the amplitude and the width of the GP pulse may be changed sequentially, that is, the time integral of the amplitude of the GP pulse may be changed sequentially. Note that the gradient axes of the selection gradient magnetic field GS, the reading gradient magnetic field GR, and the phase gradient magnetic field GP may be determined independently of the X, Y, and Z axes.

In order to obtain information emphasizing the transverse relaxation time (hereinafter referred to as T2) of the nuclear spin, the transverse relaxation time T2 of the nuclear spin differs depending on the surrounding state of the atomic nucleus, and this can be detected to obtain information on the specimen 13. In order to obtain information on the nature of the tissue, etc., first, in Figure 6, in period 1, the high frequency 90
° A pulse is applied to the subject 13 to invert the direction of the composite vector (hereinafter referred to as magnetization) of nuclear spins that was oriented in the direction of the static magnetic field (Z-axis direction) by 90 degrees with respect to the Z-axis (Fig. 8A), and this inclination Due to the inhomogeneity of the magnetic field, the directions of the nuclear spins tend to diverge in the plane perpendicular to the Z axis, that is, they dephase (FIG. 8B). High frequency 18 in period 2
A 0° pulse is applied to the subject 13 to change the magnetization direction to 180°.
Rotation (Fig. 8C) At this time, the directions of each nuclear spin, which were about to be separated in a plane perpendicular to the Z axis, are concentrated in one direction, that is, rephased, and the directions of each nuclear spin are aligned. 90 of the high frequency during period 3 in the state (Fig. 8D).
A ° pulse is applied to the subject 13 to return the direction of the nuclear spin to the Z-axis direction (FIG. 8E). The magnitude of this returned magnetization is smaller than the magnitude of the magnetization that was oriented in the Z-axis direction immediately before period 1 (FIG. 8A), and this difference is due to the magnitude of the magnetization that was oriented in the Z-axis direction immediately before period 1. This corresponds to the transverse relaxation phenomenon of the nuclear spin (transverse relaxation time T2) up to just before the pulse. After applying a spoiler magnetic field in period 4 to remove the influence of a non-uniform magnetic field, etc., the sequence starts from the period 5 described above and obtains information emphasizing transverse relaxation.

[0010] The preprocessing of the sequence for reading the NMR signal containing spatial information after period 5, that is, the high frequency pulse applied to the subject 13 during periods 1 to 3 is called a preparation pulse. Phase gradient magnetic field GP for phase encoding
When collecting NMR signals by applying a preparation pulse each time the magnitude of the image changes, that is, when repeating the sequence shown in FIG. 6, it takes a long time to obtain one screen worth of information. From this point, as shown in Fig. 9, after applying one preparation pulse, n times of read pulses are applied, and the phase gradient magnetic field GP is changed for each read pulse to collect NMR signals for one screen. It has been proposed to obtain an NMR signal of This is the snapshot flash method (Mag Res Med Magazine 13
Vol. p77-(1990)) or turbo flash method (S
MRM New York Conference Abstract p85 (19
90)).

In addition to this method, there is a stimulated echo method as a method for collecting NMR signals. this is,
As shown in FIG. 10, first, in period 1, a high-frequency 90° pulse is applied to the subject 13 to change the magnetization direction of the nuclear spins that were oriented in the direction of the static magnetic field (Z-axis direction) to the Z-axis with the X-axis as the center. Tilt by 90° (Figure 11A). Due to the inhomogeneity of the magnetic field, the directions of these fallen nuclear spins tend to become scattered within the plane perpendicular to the Z-axis, but this dephasing can be canceled as described below, so here So 9
Explain while lying down at 0°. Next, in period 2, a gradient magnetic field G is used to generate stimulated echoes.
Add ST. The gradient axis of the gradient magnetic field GST can be arbitrarily selected. FIG. 11A shows a case where a strong magnetic field is applied by GST according to the position of magnetization within one pixel in the order of A, B, and C, and multiple magnetizations within one pixel are twisted by the gradient magnetic field GST ( Figure 11B). Next, in period 3, a high-frequency 90° pulse is applied to the subject 13 to tilt the direction of magnetization of nuclear spins by 90° around the X axis (Fig. 11C).
. As a result, the vector component of magnetization in the Z-axis direction of the nuclear spin magnetization is stored in the Z-axis. The process up to this point will be referred to as the stimulated echo preparation process. Furthermore, in period 4, the magnetization of the vector component perpendicular to the Z-axis of the magnetization is canceled by the spoiling pulse GSP (FIG. 11D). After that, the repeating sequence starts from period 5 onwards. In period 5, the magnetization stored on the Z axis is tilted by α°, and in period 6, the gradient magnetic field G for rephasing is applied.
RR is applied to restore the twisted magnetization due to the gradient magnetic field GST in period 2, and a read magnetic field is applied to enable measurement to obtain the nuclear magnetic resonance signal required in period 7.

Furthermore, as described above, the dephasing that occurs from period 1 to period 3 is the time τ3 from the 90° RF pulse in period 1 to the 90° RF pulse in period 3, and the time τ3 from the read pulse in period 5 to the 90° RF pulse in period 7. Cancellation is achieved by equalizing the time to the peak of the NMR signal τ4. This stimulated echo method can cancel the inhomogeneity of the magnetic field.

The scan condition determining section 17b in the signal processing device 17 shown in FIG. 5 sends a signal for controlling the generation of the pulse sequence shown in FIG. The signal processing device 17 generally includes an electronic computer, and performs processing control using this computer. FIG. 5 functionally shows some of the functions of the signal processing device 17.

[0014]

[Problem to be solved by the invention] Although the stimulated echo method can cancel the inhomogeneity of the magnetic field,
It was difficult to obtain T2-weighted images at the same time. That is, the echo time TE, which is the sum of the time τ3 from the 90° pulse in period 1 to the 90° pulse in period 3, and the time τ4 from the read pulse in period 5 to the generation of the nuclear magnetic resonance signal in period 7, is T2. Emphasis is placed on
To obtain sufficient T2 relaxation, it is necessary to lengthen the time τ3. On the other hand, in order to cancel the influence of magnetic field inhomogeneity using the stimulated echo method, it is necessary to set τ3 = τ4, which means that the time τ3 is lengthened and τ3 = τ4.
In this case, it is necessary not only to lengthen τ3 but also to lengthen τ4 in the repeating sequence, which increases data collection time and is inefficient in terms of time.

If τ4 is shortened in order to avoid this, the benefits of the stimulated echo method cannot be obtained, and the image quality deteriorates due to the influence of magnetic field inhomogeneity. Therefore, the present invention makes it possible to obtain nuclear magnetic resonance signals using the stimulated echo method, which provides a strong T2 enhancement effect, reduces the influence of magnetic field inhomogeneity, and shortens the data collection time. The purpose of the present invention is to provide a nuclear magnetic resonance imaging device.

[0016]

[Means for Solving the Problems] This invention places an object under test in a static magnetic field, and reduces the magnetization due to nuclear spin of the object by 9
After tilting the subject to a plane perpendicular to the direction of the static magnetic field using a 0° RF pulse and applying a gradient magnetic field to the magnetization, a preparation pulse train is applied to the subject to rotate the magnetization by 90° using an RF pulse, and then a read pulse is applied multiple times. In the nuclear magnetic resonance imaging apparatus that obtains nuclear magnetic resonance information from the subject by stimulated echoes for each reading pulse given to the subject, the preparation pulse train further includes a 90° RF pulse to stabilize magnetization. It is characterized by adding a transverse relaxation emphasizing pulse train in which the magnetization is rotated by 180° by a 180° RF pulse after being tilted to a plane perpendicular to the direction of the magnetic field, and then the magnetization is returned to the direction of the static magnetic field by a 90° RF pulse. It is something.

Depending on the sequence of preparation pulses, the 90° RF pulse of the preparation pulse train and the 90° RF pulse of the added transverse relaxation emphasizing pulse train may be used in whole or in part to reduce the number of pulses of the entire preparation pulse train. reduce

[0018]

[Operation] According to the present invention, in order to obtain the T2 enhancement effect, it is only necessary to lengthen the time of the transverse relaxation enhancement process, that is, the transverse relaxation enhancement pulse train, thereby canceling out the influence of magnetic field inhomogeneity due to the stimulated echo method. Therefore, there is no need to extend the echo time during the data collection period, so the data collection time can be shortened, and T2-weighted images can be efficiently obtained even in high-speed imaging methods using stimulated echoes as preparation pulses.

[0019]

EXAMPLES The present invention will be explained in detail below based on examples. FIG. 2 is a diagram showing an embodiment of the invention. First, as in Fig. 9, a 90° pulse - 180° pulse - 90° pulse is applied to the subject to emphasize T2, and then a 90° pulse - gradient magnetic field GST - 90° pulse is applied to the subject to stabilize the spin. Create a state where simulated echo occurs. After applying this preparation pulse, reading is performed n times to collect NMR signals for one screen. Here, by setting τ1 = τ2 and τ3 = τ4, the non-uniformity of the magnetic field can be canceled. In addition, the T2 enhancement effect can be increased simply by lengthening the times τ1 and τ2 of 90° pulse - 180° pulse - 90° pulse, and 90° pulse - gradient magnetic field GST -
Time of 90° pulse τ3 and time of n readings τ4
There is no need to make it long. Therefore, in the stimulated echo method, T2 weighted images can be obtained with almost no change in data acquisition time.

FIG. 3 is a diagram showing another embodiment, and FIG.
90° pulse - 180° pulse - 90° pulse and 90
The order of ° pulse - gradient magnetic field GST - 90 ° pulse is reversed, and even in this case, the pulse time τ1
A T2-weighted image can be obtained by simply increasing τ and τ2 without increasing the pulse acquisition time. FIG. 1 is a diagram showing still another embodiment, in which two 90° pulses for the stimulated echo method and two 90° pulses for T2 emphasis are combined, and compared to the preparation pulse described above. The number of pulses can be reduced. That is, a 90° pulse is applied to the subject during period 1, and Z
Spins that are aligned parallel to the axis are turned 90 degrees. The defeated spins are dephased, but in period 2 after τ1 180
When the ° pulse is applied, the spins align again in the direction perpendicular to the Z axis after τ2 (τ2 = τ1). 180°
A gradient magnetic field GST is applied in period 3 τ2 after the pulse, and a 90° pulse is further applied in period 4 after τ3 (τ3 = τ4) to prepare for generating a stimulated echo. Spins oriented in directions other than the Z-axis are dispersed by the spoiling pulse GSP so that no signal is generated. After applying the preparation pulse as described above to the subject, reading is performed n times to collect NMR signals for one screen. In this example as well, the time from period 1 to period 3 (τ1 +
The T2 emphasis effect can be obtained during the period τ2 ), and the T2 emphasis effect can be increased simply by lengthening this (τ1 +τ2). Note that periods 2 and 3 are swapped, a gradient magnetic field GST is applied to period 2, and a 180° magnetic field is applied to period 3.
Pulses may also be applied. In that case, the period between the 90° pulse of period 1 and GST is set as τ3, the period between GST and the 180° pulse is set as τ1, and the period between the 180° pulse and the 90° pulse of period 4 is set as τ2.

FIG. 4 shows another embodiment of the present invention, in which the pulse sequence from period 1 to period 3 is a 90° pulse-gradient magnetic field GST-90 similar to the embodiment in FIG.
°Apply a pulse to the subject. In other words, the gradient magnetic field GST
The spin is dephased on a plane perpendicular to the Z-axis, and by applying the 90° pulse in period 3, the magnetization is divided into a vector component in the Z-axis direction and a vector component perpendicular to the Z-axis. In this embodiment, information on spins oriented perpendicular to the Z-axis is detected. Spins that are oriented perpendicular to the Z-axis due to the 90° pulse applied in period 3 are reversed by the 180° pulse in period 4 after τ1, and are oriented perpendicular to the Z-axis by the 90° pulse in period 5 after τ2. Directed. 90 of period 5
°The rotation axis of the pulse is shifted by 90 degrees from the rotation axis of the RF pulse from periods 1 to 4. For example, if the rotation axis of the RF pulse from period 1 to period 4 is the X axis, then the rotation axis of the RF pulse from period 1 to period 4 is 90 degrees. The Y-axis is used as the axis of rotation by the pulse. The axis of rotation by the RF pulse can be easily changed by changing the phase.

[0022] Also in this embodiment, from period 1 to period 3
A 90° pulse-gradient GST-90° pulse of up to 90° places the spins in such a state that a stimulated echo occurs, and during the next τ1 + τ2 T2
A highlighting effect can be obtained. Note that the spins directed to the Z axis by the 90° pulse in period 3 are
The ° pulse is directed perpendicular to the Z axis, but is dispersed by the spoiling pulse GSP so that no signal is generated.

In the above description, since the read pulse tilts the nuclear spin by α°, the magnetization in the Z-axis direction becomes COS α° times that before the read pulse is applied, and the NMR signal obtained with each read pulse gradually becomes smaller. For this reason, the angle α° of inclination by the pulse may be gradually increased for each reading pulse. In addition, in the above, one screen worth of NMR signals was collected for each preparation pulse, but one screen worth of NMR signals was collected for each preparation pulse.
The MR signal may be divided into a plurality of parts and collected, and a preparation pulse may be provided for each collection, that is, the NMR signal for one screen may be collected with a plurality of preparation pulses. Furthermore, the present invention can also be applied to a so-called multi-slice method in which NMR signals are collected in other slice planes using the waiting time for relaxation recovery. The change in strength of the gradient magnetic field GP for phase encoding is
It does not necessarily have to be changed gradually, such as by alternately taking positive and negative values. The present invention can also be applied to the collection of three-dimensional image data, and can be applied not only to the collection of image data but also to the collection of spectral information.

[0024]

According to the present invention, in a nuclear magnetic resonance imaging apparatus using a preparation pulse, the effect of a non-uniform magnetic field can be canceled out by the stimulated echo method, and a sufficient T2-weighted image can be obtained. In this case, the time of the repeated reading sequence does not need to be extended, and time efficiency does not deteriorate.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of an NMR signal acquisition sequence in an embodiment of the present invention.

FIG. 2 is a diagram showing an example of an NMR signal acquisition sequence in another embodiment of the invention.

FIG. 3 is a diagram showing an example of an NMR signal acquisition sequence in still another embodiment of the present invention.

FIG. 4 is a diagram showing an example of an NMR signal acquisition sequence in still another embodiment of the present invention.

FIG. 5 is a block diagram showing a general configuration example of a nuclear magnetic resonance image information acquisition device.

FIG. 6 is a diagram showing a conventional acquisition sequence of transverse relaxation-enhanced NMR signals.

FIG. 7 is a diagram for explaining a slice plane of a subject, frequency encoding, and phase encoding.

FIG. 8 is a diagram showing the behavior of a nuclear spin synthesis vector due to a preparation pulse with transverse relaxation emphasis.

FIG. 9 is a diagram showing a sequence of a flash method using a conventional preparation pulse.

FIG. 10 is a diagram showing an NMR signal acquisition sequence using a preparation pulse in a conventional stimulated echo method.

FIG. 11 is a diagram showing the behavior of a nuclear spin synthesis vector due to a preparation pulse in a conventional stimulated echo method.

[Explanation of symbols]

GST Gradient magnetic field for stimulated echo GSP Gradient magnetic field for spoiling GRR
Gradient magnetic field Gs for rephasing Gradient magnetic field GSR for slice selection
Gradient magnetic field for rephasing GR Gradient magnetic field for reading GRd Gradient magnetic field for dephasing GP Gradient magnetic field for phase

Claims (3)

[Claims] Claim 1: A subject is placed in a static magnetic field, magnetization due to nuclear spins of the subject is tilted to a plane perpendicular to the direction of the static magnetic field by a 90° RF pulse, and a gradient magnetic field is applied to the tilted magnetization. after that, means for applying to the subject a preparation pulse train that rotates the magnetization by 90 degrees with an RF pulse; and after applying the preparation pulse train, applying a read pulse a plurality of times to the subject;
In the nuclear magnetic resonance imaging apparatus, the magnetization is changed in the direction of the static magnetic field by a 90° RF pulse in the preparation pulse train. and a means for adding a transverse relaxation emphasizing pulse train that rotates the tilted magnetization at the rear by 180° by a 180° RF pulse, and then returns the magnetization to the direction of the static magnetic field by a 90° RF pulse. A nuclear magnetic resonance imaging device characterized by: 2. Two 90° RF pulses for obtaining the stimulated echo and two 90° RF pulses for the transverse relaxation-enhanced pulse train, and between these 90° RF pulses, the slope Applying a magnetic field and performing the above 18
2. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the 0° RF pulse is positioned at an interval. 3. One of the pulse train for obtaining the stimulated echo and the transverse relaxation-enhanced pulse train is temporally earlier, and the later 90° RF pulse in the temporally earlier pulse train is temporally earlier. 2. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the RF pulse is also used as a previous 90° RF pulse in a subsequent pulse train.